1. Field of the Invention
This invention relates to a method of treating low-level radioactive waste discharged from, for example, an enriched uranium conversion process.
2. Description of the Prior Art
An enriched uranium oxide is used as atomic fuel for a light water reactor. As natural uranium contains only about 0.7% of .sup.235 U which contributes to nuclear fission, it is usual practice to convert a natural uranium oxide to UF.sub.6, enrich UF.sub.6 by, for example, gaseous diffusion or centrifugal separation so that it may contain about 3% of .sup.235 U, and reconvert the enriched UF.sub.6 to UO.sub.2.
The following methods are known for the wet reconversion of enriched UF.sub.6 to UO.sub.2 :
(1) UF.sub.6 is blown into an aqueous solution of aluminum nitrate for hydrolysis, and pure uranyl nitrate [UO.sub.2 (NO.sub.3).sub.2 ] is obtained by solvent extraction. Ammonia is added to an aqueous solution thereof to form ammonium diuranate (ADU) [(NH.sub.4).sub.2 U.sub.2 O.sub.7 ]. Ammonium diuranate is separated and calcined to form U.sub.3 O.sub.8, and U.sub.3 O.sub.8 is reduced in a hydrogen atmosphere to form UO.sub.2 powder. PA0 (2) Uranyl fluoride (UO.sub.2 F.sub.2) is obtained by the hydrolysis of UF.sub.6 in water, and ammonia is added to uranyl fluoride to form ammonium diuranate. It is calcined to form U.sub.3 O.sub.8 and U.sub.3 O.sub.8 is reduced to UO.sub.2. PA0 (3) Uranyl fluoride is obtained by the hydrolysis of UF.sub.6 in steam, and CO.sub.2 and ammonia are added to UO.sub.2 F.sub.2 to form ammonium uranyl tricarbonate (AUC) [(NH.sub.4).sub.4 (UO.sub.2)(CO.sub.3).sub.3 ]. It is calcined to form U.sub.3 O.sub.8 and U.sub.3 O.sub.8 is reduced to UO.sub.2.
The precipitated ammonium diuranate or ammonium uranyl tricarbonate is recovered by filtration, and the filtrate remaining thereafter is low-level radioactive waste. Standards are specified by law for discharging low-level radioactive waste from the system, and classified by nuclear species.
As the majority of enriched UF.sub.6 presently used in Japan is obtained from natural uranium, all of the nuclear species which the low-level radioactive waste resulting from its reconversion contains are known, and the waste fully satisfies the standards for its discharge. If the uranium recovered by the reprocessing of spent fuel is used as a part of source material, however, it is possible that the low-level radioactive waste resulting from the reconversion of enriched UF.sub.6 may have a higher radioactive concentration. Although the radioactivity of low-level radioactive waste has so far not presented any particular problem, a possible increase in the amount of uranium recycled from the reprocessing of spent fuel makes it urgently necessary to establish a method for removing radioactive nuclear specis from low-level radioactive waste.